1. Field of the Invention
This invention relates to starter units, and more particularly to a starter unit which is used to start the engine of a vehicle, having a rotary output shaft which is driven by an electric motor and is movable axially.
2. Description of the Prior Art
A typical example of a conventional starter unit having a rotary output shaft which is axially movable is a so-called coaxial starter unit which is as shown in FIG. 2.
The conventional coaxial starter unit, as shown in FIG. 2, includes a DC motor which has a yoke 1 forming a magnetic circuit and serving as an outer wall of the starter unit, permanent magnets 2 arranged on the inner surface of the yoke 1 at equal angular intervals, an armature 3 rotatably mounted along the central axis of the yoke 1, and a commutator (not shown) provided at one end of the armature 3.
In the DC motor 4, the armature 3 is made up of a hollow rotary shaft 5, and an armature core 6 mounted on the armature rotary shaft 5. A rotary output shaft 7 is provided on one side of the DC motor 4 (on the right side in FIG. 2) in such a manner that it is in alignment with the armature rotary shaft 5. The rotation is transmitted with the aid of a drive force transmitting device 8. The drive force transmitting device 8 has: a planetary gear speed reducer 9, over-running clutch 10 and a helical spline 7a formed on the rotary output shaft 7 in such a manner that it is engaged with a second helical spline 10d which is formed in the inner cylindrical wall of the clutch inner component 10a of the over-running clutch 10. The hollow rotary shaft 5 of the armature has an internal passageway 5a formed in it in such a manner that it is extended along its axis. One end portion of the output shaft 7 is in alignment with the armature rotary shaft 5 and is inserted into the internal passageway 5a together with a metal sleeve in such a manner that the output shaft is axially slidable through the metal sleeve.
The torque of the armature rotary shaft 5 is transmitted to the output shaft 7 through the planetary gear speed reducer 9 and the over-running clutch 10. The speed reducer 9 has: a sun gear 9a which is integral with the front end portion of the armature rotary shaft 5, an internal gear 9b formed around the sun gear 9a, and a plurality of planetary gears 9c which are engaged with the sun gear 9a and the internal gear 9b, and are rotatably supported by supporting shafts 9c which are secured to the clutch outer component 10b of the over-running clutch 10. The clutch inner component 10 of the over-running clutch 10 is engaged with the helical spline 7a which is formed in the outer cylindrical wall of the large diameter portion 7b of the output shaft 7. Therefore, the output shaft 7 is slid axially by the torque applied thereto through the clutch component inner 10a. As the output shaft 7 is slid in this manner, a pinion 11 mounted on the front end portion of the output shaft 7 are protruded from an outer front bracket 12 (not illustrated) to engage with the ring gear (not shown) of the engine to rotate it. In FIG. 2, reference character 10c designates rollers interposed between the clutch outer component 10b and the clutch inner component 10a. The aforementioned internal gear 9b is formed in a gear forming member 13, which serves as an inner front bracket and is coupled to the yoke 1 with through-bolts 14. The through-bolts 14 are further used to mount an electromagnetic switch 15 is mounted on the rear end of the DC motor 4. The electromagnetic switch 15 is used to slide the output shaft 7 axially, and is employed to supply current from the battery to the DC motor 4 when the vehicle's key switch is turned on.
The conventional coaxial starter unit thus constructed suffers from the following difficulties. If the starting operation is carried out during inertial rotation of the engine, then excessively an large load is applied to the power transmitting system, especially to the ring gear, so that the pinion 11 and other components may be damaged.
When such an abnormally large impact is made on the power transmitting system, damage may be prevented by a method in which the carrier of the planetary gear speed reducer and the clutch outer component of the over-running clutch are engaged with each other in such a manner that they slide with a predetermined rotation torque. However, in this case, the engagement must serve as a so-called "torque limiter" which positively and accurately cuts the transmission of power attributing to the impact load only. For this purpose, it is necessary to set a relatively high transmission torque for the slide surface of the carrier and clutch outer thus engaged.